Conical separation discs have been known for more than 100 years (see for example DE 48615) and have been extensively used in many types of centrifugal separator. Despite it also being about 100 years since it was proposed to provide a separation disc with elevations formed integrally with it (see for example SE 21700 and U.S. Pat. No. 1,006,622), this technique is nevertheless not applied in practice when thin sheet metal is used as material for separation discs. In contrast, it has been extensively possible to produce separation discs made of plastic which are provided with integrally formed spacing means. This is easy to do, since separation discs made of plastic can be manufactured by injection moulding technology.
Separation discs made of thin sheet metal are usually produced by spinning and are provided with spacing means in the form of narrow strips or small circles of sheet metal which are fastened to the separation discs after the forming in various ways, usually by spot welding. An operation for attaching separate spacing means to separation discs, e.g. by welding, is both expensive and time-consuming.
A method for pressing separation discs with integrally formed elevations is referred to in DE 197 05 704, which involves flow forming of a sheet metal blank over a cone to produce a separation disc with integrally formed elevations. The sheet metal blank is pressed over the cone by means of roll. The cone is provided with depressions which during the flow forming of the sheet metal blank become filled with material from the blank.
In making separation discs with elevations according to the method referred to in DE 197 05 704, it may be difficult to achieve a separation disc on which the elevations exceed a certain height. Trying to achieve this may cause unduly stress on the material especially if the blanks are thin, because of the amount of material needed to fill the depressions on the pressure roll cone in relation to the “available” material of the blank or the material available is simply not enough to fill the depressions. This is especially the case if the ratio between the thickness of the separation disc and the height of the elevations or spacing members exceed 1 which is desired in some applications.